I think the way this is written is what makes it look so complicated. They define "inc" as knitting into the front and back of the next stitch. Brackets indicate a stitch sequence that is repeated and have nothing to do with the size. So for your granddaughter's small poncho the row should go:
[inc, place marker, P1, K1, P1, place marker, inc] repeat between brackets twice more, then inc, place marker, P1, K1, P1, place marker, inc – 28 sts.

The bracketed section brackets uses 5 of your original stitches and ends up being 7 sts. Then you just repeat to the end of the row.